This invention relates to a load sense hydrostatic vehicle steering system that includes a fluid source which can deliver variable amounts of fluid (e.g., a variable displacement pump, a fixed displacement pump with a priority valve) and a hydrostatic steering controller which effects steering and also forms part of a pilot circuit for controlling the fluid flow delivered from the source.
In a load sense hydrostatic vehicle steering circuit, when an operator steers, a hydrostatic steering controller receives fluid from a pump, meters the fluid, and directs metered fluid to a steering motor to effect steering of the vehicle. The amount of fluid which is needed to effect steering depends on the rate at which the operator steers (operator demand) and the amount of resistance encountered by the vehicle's wheels (steering load). When the operator is not steering, only a standby amount of fluid needs to be maintained in the steering circuit. Excess fluid, beyond that needed in the steering circuit, can be used for operating auxiliary devices on the vehicle. In most load sense vehicle steering systems, a priority valve operates to assure priority fluid flow to the steering circuit, and to direct excess fluid to an auxiliary circuit where it is available to operate auxiliary devices.
In a system in which the pump has a variable displacement, the pump's maximum displacement is high enough to deliver more fluid flow and pressure than is necessary for making the most demanding of steering maneuvers. When the operator is not steering, or is effecting a steering maneuver which does not require a high flow and pressure, the pump displacement is reduced and/or the priority valve is positioned to reduce the amount of fluid in the steering circuit to a predetermined standby level. When the fluid in the steering circuit is at a standby level, and the operator makes a steering maneuver requiring a higher fluid flow and pressure, the pump displacement is increased, and/or the priority valve is positioned, to bring fluid flow and pressure in the steering circuit to levels which are sufficient to effect steering as demanded by the operator.
U.S. Pat. Nos. 3,931,711 and 4,079,805 show load sense hydrostatic vehicle steering systems in which a hydrostatic steering controller effects steering of the vehicle and also forms part of a fluid control circuit which controls the displacement of a variable displacement pump. When there is no steering, the fluid control circuit sets the displacement of the pump at a minimum level in which the pump maintains fluid in the steering circuit at a standby level. When an operator steers, the fluid control circuit operates to increase pump displacement, when necessary, to insure that the fluid delivered from the pump will be sufficient to effect steering as demanded by the operator.
In the systems shown in U.S. Pat. Nos. 3,931,711 and 4,079,805, the steering controller includes a steering control valve and a positive displacement metering unit. The controller has an inlet port which is connected to the variable displacement pump, and the steering control valve is biased to a neutral position in which it either totally blocks fluid at the controller's inlet port, or directs a small amount of pilot fluid to a reservoir. Thus, the controller is basically "closed center", when the steering control valve is in its neutral position. In response to a steering effort, the steering control valve moves away from its neutral position. After a predetermined range of movement away from its neutral position, the steering control valve reaches an operating condition in which it directs fluid from the controller's inlet port to the metering unit and directs metered fluid from the metering unit to the steering motor. More specifically, when the steering control valve reaches an operating condition, it establishes a main flow control orifice and directs fluid from the inlet port through the main flow control orifice to the metering unit. During steering, when the steering control valve is in an operating condition, the steering control valve has an operating range of movement (relative to the neutral position) in which the flow area of the main flow control orifice varies in accordance with the demand for fluid to effect steering. Specifically, when the steering control valve is in the operating range, the flow area of the main flow control orifice varies directly with variations in (i) the rate at which the operator steers (operator demand) and/or (ii) the amount of resistance encountered by the vehicle's wheels (steering load). In response to a high enough steering demand, the steering control valve will move through its full operating range to a point at which the main flow control orifice will have a fixed, maximum flow area.
In certain of the systems shown in U.S. Pat. Nos. 3,931,711 and 4,079,805, a pressure signal, taken from the downstream side of the variable main flow control orifice, is directed to a load sense port in the controller and used to control the displacement of the pump. Specifically, while the steering control valve is in its operating range, the area of the main flow control orifice, and thus the pressure signal at the load sense port, will vary in accordance with the operator's demand and the steering load. As the pressure signal varies, it varies the displacement of the pump so that the pump delivers an appropriate amount of fluid to effect steering as demanded by the operator. In other systems shown in those patents, a variable area orifice is provided in a pilot circuit which is in parallel with the main flow control orifice. The pilot circuit orifice also varies in area according to the operator demand and/or the steering load, at least over the operating range of the steering control valve. A pressure signal taken from one side of the pilot circuit orifice is directed to the load sense port of the controller and is used to control the displacement of the pump.
In the systems of U.S. Pat. Nos. 3,931,711 and 4,079,805, the fluid source is a variable displacement pump. The principles of the patents are equally applicable for controlling a priority valve, however, as would have been readily obvious to one of ordinary skill.
A further development in load sense hydrostatic steering systems is shown in U.S. patent application Ser. No. 243,497. According to U.S. application Ser. No. 243,497, a steering system may include from a variable displacement pump for directing fluid, through a priority valve, to a steering controller. The steering controller is basically "closed center", but does direct a small amount of pilot fluid to a reservoir, when there is no steering. Specifically, a pilot fluid circuit originates outside the steering controller and communicates with the load sense port of the steering controller. When there is no steering, the steering control valve directs the pilot fluid from the load sense port to the reservoir. Pressure signals, taken at certain points in the pilot fluid circuit, act on the priority valve and on a flow compensator valve which controls the displacement of the pump. The pressure signals determine a standby position for the priority valve and a standby displacement for the variable displacement pump, in order to maintain standby levels of fluid in the system.
In the preferred form of the system according to U.S. application Ser. No. 243,497, when steering is initiated, the steering control valve abruptly restricts the pilot flow. The restriction occurs even before the valve reaches an operating condition in which it establishes the main flow control orifice that directs flow from the inlet port to the metering unit. The abrupt restriction of pilot flow produces a pressure spike in the pilot circuit. The pressure spike acts on the priority valve and on the flow compensator valve which controls the displacement of the pump to (i) urge the priority valve rapidly to its priority position and (ii) urge the flow compensator valve to a position in which the displacement of the pump increases. As steering effort continues, and the steering control valve establishes a flow through the variable main flow control orifice, the pressure in the pilot circuit is controlled by the pressure at the downstream side of the main flow control orifice. The pressure at the downstream side of the main flow control orifice is used to control the position of the priority valve and the flow compensator valve in accordance with the operator's demand and the steering load.
In load sense systems which are basically "closed center", but which also direct a pilot flow of fluid to a reservoir (e.g., the system of U.S. application Ser. No. 243,497), it is desirable to minimize the amount of pilot fluid directed to the reservoir when there is no steering. This is because whatever pilot fluid is directed to the reservoir cannot be used to operate auxiliary devices carried on the vehicle. Ideally, by employing the principles of U.S. Pat. Nos. 3,931,711 and 4,079,805, and the further principles taught by U.S. application Ser. No. 243,497, only a very small amount of pilot fluid (e.g., 0.25 gal/min) should be directed to the reservoir. The pilot flow to the reservoir will be just enough to (i) maintain a minimum standby fluid pressure level in the steering circuit when there is no steering and (ii) effect a rapid increase fluid flow from the source to the steering circuit during steering, and thereby insure that during steering, there will be adequate fluid flow and pressure in the steering circuit to effect steering as demanded by an operator. In such a system, a variable displacement pump could have a very low displacement position when the system is in standby, and be brought up from its low displacement position when there is a need for flow and pressure in the steering circuit.
Experience has shown that some variable displacement pumps react slowly when the system described above tries to bring them up from a very low displacement position. The inertia of the components which have to initially move in order to increase the pump's displacement, and thereafter the compressibility of the working fluid (e.g. oil), are the primary factors in delaying the pump's reaction time. Further, some large area priority valves also have a high inertia, which can make it difficult to get the priority valves to respond to steering as quickly as is desirable. Since load sense systems are expected to react to steering demands in fractions of a second, a fluid source whose response is delayed for even a very small time beyond that which is intended may be unsuitable for use in a load sense system.